One of my new favorite plugins for WordPress is Really Simple SSL. Our site evOwl.com was loading the logo from a non secure URL and causing a mixed content error. To fix this we installed Really Simple SSL WordPress plugin and activated it. It worked great with no additional configuration required. They offer a premium version with support and some other advanced features such as HTTP Strict Transport Security and a pre install scanner.
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If you want to find what is causing your mixed SSL content error, visit our guide on how to identify the problem.